Several days ago, Rick DeMeis posted information about a new intelligent high-side switches for automotive applications from International Rectifier that caught my eye. I was curious about these parts because, such current monitoring functions (high-side drive, ground-referenced measurement) must operate a scaled pair of transistors in a matched electrical environment despite the disparity in their output potentials. So I downloaded the datasheet to take a closer look.

I should mention here that there are two errors in the datasheet schematics. International Rectifier may (or may not) have posted revised documentation by the time you get involved with the part, but in case they have not, explanations appear below.

The switch function is simple enough, as shown in the application schematic (Figure 1). The external NPN transistor pulls the input low to turn on the high-side switch. Load current flows from the OUT pin and a scaled copy of that current flows from the Ifb pin.

Figure 1

As drawn, however, the application schematic has an error: Between the control signal source and the NPN’s base there should be a resistor to set the NPN’s base current (Figure 2). Without the resistor, the base current will be roughly equal to the short-circuit current of the control source and, should that be a robust signal, the NPN transistor could be short lived. You can either add the base resistor, or replace the NPN with an N-Channel MOSFET. The International Rectifier applications engineer I discussed this with said that either approach would work but that it was likely that the MOSFET would cost more than the NPN with its base resistor.

Figure 2

Once the device processes its input signal, a FET driver, operating with the aid of a charge pump provides the gate voltage necessary to turn on both output MOSFETs (Figure 3).

Figure 3

The two output FETs (Figure 4) are scaled copies of one another and are, no doubt, arranged on the IC’s layout to maintain good thermal communication.

Figure 4

The two devices’ drain and gate contacts are common to one another but in order to maintain proper current scaling, their source potentials must agree as well, despite the fact that one is connected to the high-side of a load while the other is referred to ground through a resistor. To force the two source potentials to agree, a P-Channel MOSFET is connected in series with the Ifb output (Figure 5). An operational amplifier measures the two source potentials and drives the P-Channel MOSFET to make the two agree. In effect, the opamp drives the P-Channel device so that it stretches across the potential difference between the two output pins. Simple, and evidently effective.

Figure 5

By the way, for those trying to understand the control switching, the second documentation error is the depiction of a 3V potential difference between the device’s positive rail and the control-comparator’s non-inverting input (Figure 6). It is drawn backwards relative to the circuit’s wiring.

Figure 6